Solid fuel boilers are commonly used by industry and utilities to generate steam for process requirements and to generate electricity. These boilers burn bark, coal, sludge, wood waste, refuse, tires, and other organic materials, often in combinations, and with fossil fuels. Generally, the organic materials have high moisture contents and are stored outdoors where they are often wet from rain water or, in the case of sludge, reclaimed from wastewater treatment facility.
In some cases, some of the moisture is removed before the fuel is delivered to the boiler by means of mechanical presses or drying chambers using hot gases from the discharge of the boiler. For example, U.S. Pat. No. 3,976,018 to Boulet teaches a fuel dryer for bagasse fuel. The dryer is separate from the boiler and uses alternating fixed and rotating conical trays over which the fuel passes downward while stack gases pass over the fuel to dry it. The fuel empties into a hopper for transporting to other equipment. U.S. Pat. No. 6,532,880 to Promuto teaches a system for drying sludge, including a shaftless spiral feed screw for moving sludge through a drying chamber as a high energy inductor draws hot gases through the chamber to dry the sludge as it advances through the chamber. U.S. Pat. No. 4,635,379 to Kroneld describes a dryer in which fuel travels on a conveyor bed while steam moves through the fuel from underneath. U.S. Pat. No. 4,254,715 to LaHaye et al. teaches a drying system in which heated air passes over the fuel in a combustion chamber, as burning occurs substantially at the bottom of the pile of fuel.
These drying systems are often troublesome, expensive, risky, and not particularly efficient. With many of these methods, the fuels still contain undesirably high moisture content. In any case, essentially all of the water in the fuel must be removed during the combustion process and when the fuels are wet the combustion process can be unstable and inefficient. The situation is further exacerbated by variable moisture contents of the fuel (e.g., from rain) that introduces variations into the combustion process and makes it more difficult to operate the boilers. Solid fuel boilers are typically constructed as large boxes (up to 100 square meters or more in floor area) with heavy steel tubing forming the walls of the box, typically referred to as the front, sides, and rear walls. The tubing typically has an outer diameter of 63.5 mm or 76.2 mm and is arrayed in parallel relationship forming flat panels, with the tubes running vertically. The tubes are typically spaced apart about 10-12 mm, with a steel membrane or fin bridging the gap. The whole assembly is seal welded together forming an air tight structure. The boiler walls, or tube panels, run vertically to the top of the boiler, up to 30 m or more tall. The walls are fed re-circulating water by headers at their lower extremity. Typically the front wall tubes are bent over more or less horizontally to form the roof of the boiler and the side walls end in relieving headers feeding back to a steam drum. The rear wall either ends in a header or feeds directly into the steam drum. In order to feed fuel and combustion air into the boiler, and to provide openings for other purposes, the boiler tubes are bent to spread them apart to form openings in the tube panel. The bottom of the boiler may be arranged to include a combustion support, such as a grate, a fluidized bed, or other arrangement. Grates include traveling grates, vibrating grates, tilting grates, and hydro-grates. Typically, the grates cover the bottom of the boiler and are made of heavy cast iron components with slots for combustion air to rise through the grate from a plenum below. The solid fuel lands on the grate and burns there. The ash is dumped off of the grate as the grate moves (rotates like a tank tread), vibrates, or tilts (in sections). Fluidized beds generally have a mass of sand or other media through which a stream of air or boiler flue gas is percolated to fluidize the bed. The fluidized bed acts as a heat sink, fuel drying system, turbulent fuel/air mixing system, fuel distribution system, and means for separating fuel and ash in the boiler. Additional combustion air ports, typically called “over fired air” (OFA) are arranged to blow air in above the grate or fluidized bed to help complete the combustion. In all of these arrangements, excessive moisture in the fuel causes poor combustion, which can result in poor operational efficiencies and high environmental emissions. The volume contained within the boiler walls is referred to herein as the “combustion chamber.” The region where most of the solid fuel burns, that is, on the grate or at the fluidized bed, is referred to herein as the “combusting zone.” It is understood that combustion of airborne combustible matter also takes place in the combustion chamber outside of the combusting zone.
In common practice the solid fuel is fed by gravity through large chutes, steeply mounted and having a cross section of about 500 mm square, from a hopper and/or conveyor system above, to the lower portion of the boiler just above the grate or fluidized bed. There are typically multiple chutes penetrating a wall or walls of the boiler. A solid fuel distributor is often integral with and at the bottom of the chute, right at the interface with the boiler wall. Mechanical distributors and pneumatic distributors are commonly used. Grate type boilers generally require some type of fuel distribution whereas fluidized bed boilers can be run without them as the fluidized bed can distribute the fuel, albeit inefficiently. Typically, the fuel slides down the chute and enters the boiler with high residual moisture content (up to 50% or more). The water in the fuel inhibits the combustion in the furnace, often requiring the continual use of supplemental fossil fuels to provide additional heat to compensate for the moisture. It is also very common for the load rate on these boilers to change frequently in reaction to changing steam demands. Inconsistent and high moisture content of the fuels makes it difficult for the boiler to respond effectively to the required load changes. This requires, again, the use of supplemental fossil fuels to improve the response of the boiler to load rate changes. Fossil fuels are typically used to start these boilers but continual use of fossil fuels is extremely expensive. Fluidized beds can help to compensate for varying moisture contents and load rates because they act as heat sinks, but they can have significant operational and mechanical problems such as sand sintering and sand erosion and they require a sand reclamation system. There is great demand for a simple means to dry solid fuels so that they are delivered to the boiler combustion chamber ready to burn. Such a system is preferably inexpensive to install and operate, reliable, effective, and safe. Various embodiments of the present invention, described below, address one or more of these challenges and provide one or more, and preferably all of these advantages.